1. Field of the Invention
The present invention relates to a light-emitting device and an electronic appliance each having a light-emitting element that utilizes electroluminescence.
2. Description of the Related Art
Compared with inorganic compounds, organic compounds provide materials that have a variety of structures and make it possible to synthesize materials that have a variety of functions depending on molecular designs. Because of these advantages, photo electronics and electronics using functional organic materials have been attracting attention in recent years.
Examples of electronic devices in which organic compounds are used as functional organic materials include solar cells, light-emitting elements, organic transistors, and the like. These are devices in which electric properties and optical properties of organic compounds are utilized. In particular, tremendous progress in light-emitting elements has been made.
It is said that a light-emitting element has a mechanism of light emission as follows: by application of a voltage between a pair of electrodes with a light-emitting layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined with each other in an emission center of a light-emitting layer to form molecular excitons, and the molecular excitons release energy when they relax to a ground state; accordingly light is emitted. A singlet excited state and a triplet excited state are known as excited states, and it is considered that light can be emitted through either excited state.
Further, in such a light-emitting element, since the pair of electrodes and the light-emitting layer are formed as films, surface light emission can easily be achieved by forming a large-area element. Since this is a feature which is difficult to obtain in light sources such as incandescent lamps and LEDs (point light sources) or fluorescent lamps (line light sources), the above-described light-emitting element has a high utility value as a light source such as a lighting apparatus.
Considering these fields of application, it can be said that the development of a light-emitting element such as that described above which is a white light-emitting element is an important subject. If a white light-emitting element with sufficient luminance, emission efficiency, element life, and chromaticity can be obtained, by combining it with a color filter, a quality full color display can be manufactured. Further, application to white light sources such as backlights and lighting apparatuses can also be considered.
For example, research and development of a white light-emitting element that has an emission spectrum with peaks in the wavelength ranges of red, green, and blue (hereinafter referred to as “3-wavelength-type white light-emitting elements”) is also underway (e.g., see Non-Patent Document 1 and Non-Patent Document 2). In Non-Patent Document 1, a structure in which three light-emitting layers, a red one, a green one, and a blue one, are layered is described. In Non-Patent Document 2, a structure in which light-emitting materials which exhibit red, green, and blue light emission are included in the one light-emitting layer is described.
However, it cannot be said that these 3-wavelength white light-emitting elements have characteristics sufficient for practical use in terms of emission efficiency and element life, and greater improvements of them are necessary. Further, it is known that light-emitting elements such as the one described in Non-Patent Document 1 cannot emit stable white light in many cases because, for example, the spectrum varies depending on the density of the flowing current.    [Non-Patent Document 1] J. Kido (and two other authors), Science, vol. 267, pp. 1332-1334, 1995.    [Non-Patent Document 2] J. Kido (and two other authors), Applied Physics Letters, vol. 67 (16), pp. 2281-2283, 1995.